1. Field of the Invention
The present invention relates to an emissive material for use in a vapor discharge device, and to a vapor discharge device having an arc tube which includes electrodes therein coated with such emissive material
2. Description of the Prior Art
The present invention will be described herein in the context of a high pressure sodium vapor discharge lamp. However, the scope of the present invention is not limited to such lamps but also covers other vapor discharge devices such as, without limitation, HCRI sodium, unsaturated vapor sodium, fluorescent, high pressure mercury, and other alkali metal lamps. Such lamps are known in the art. For example, high pressure sodium lamps containing low or unsaturated fills of sodium and mercury are known to the art, as are lamps which use electrodes that include thorium oxide, yttrium oxide, oxide compounds containing the oxides of barium, calcium, tungsten, and yttrium, and oxide compounds containing strontium and yttrium oxides. Such lamps have frequently suffered from a loss of sodium as a constituent of the arc stream which is confined within the arc tube during operation of the lamp. The loss of this sodium reduces the luminance of the lamp.
Examples of current art emission mixtures for high pressure sodium lamps include dibarium calcium tungstate as described in U.S. Pat. No. 3,708,710, yttrium oxide as described in Japanese Patent No. 62-82640, thorium oxide as described in U.S. Pat. No. 3,919,581, strontium yttrium oxide as described in European patent application EP 0159 741, tribarium diyttrium tungstate as described in U.S. Pat. No. 4,152,619, and a reacted mixture of barium zirconate and strontium zirconate as described in U.S. Pat. No. 4,210,840. An example of current art emission mixes for high pressure mercury lamps contains barium-calcium-hafnium carbonate-oxide mixtures as described in U.S. Pat. No. 4,044,276. All of the foregoing materials exhibit several problems. For example, yttria emission materials have a high work function and operate at high electrode temperatures. Dibarium calcium tungstate and tribarium diyttrium tungstate are reactive with sodium in unsaturated vapor lamps. Thorium oxide is radioactive which poses health problems. The barium-calcium-hafnium oxide mixtures are somewhat reactive with the ambient atmosphere and release water and carbon containing gases into the lamp during manufacture which react with tungsten electrode structures. The strontium yttrium oxide compound shows electrode voltage rise and lumen loss with life.
A more recent effort to reduce the rate of sodium loss from an arc tube in a high pressure sodium vapor discharge device is set forth in U.S. Pat. No. 4,806,829 which is assigned to the same assignee as the present application. This patent teaches the use of an emission material which includes an oxygen getter, such as, zirconium and/or niobium, and thorium dioxide.
It is an object of the present invention to overcome the disadvantages of such conventional emissive material and to provide an emissive material which is highly refractory and has an excellent electron emitting activity, very low sodium reactivity, low operating temperature, good starting characteristics, low initial deterioration of the D line such that the sodium D line remains high for the life of the lamp, low electrode voltage, good lumen maintenance, and ease of manufacture.